This is some sort of old DB corruption - the revision database thinks the page is a Flow board, but the Page database thinks its wikitext. Probably related to T337807

I suspect someone who has delete access there can just delete the page, which will resolve the inconsistency and turn it into a deleted (and probably empty with zero topics) Flow board.

(Note that Flow has a bug that viewing a deleted Flow board often doesn't work - it might be worth looking at the Flow database to determine whether it knows about a board with that title. Also this is the talk page of what seems to be the sandbox, so there probably isn't any useful content anyway)

If you go to https://ca.wikipedia.org/wiki/Especial:ChangeContentModel/Viquip%C3%A8dia_Discussi%C3%B3:Guia_b%C3%A0sica/Proves and switch it to plain text and back it should "just work", depending on what's actually there.

See the log of this page: https://ca.wikipedia.org/wiki/Especial:Registre?page=Viquip%C3%A8dia+Discussi%C3%B3%3AGuia+b%C3%A0sica%2FProves. It was inadvertently moved along with the associated page. Probably its content is a redirect while the content model was not changed. I confirm it is a sandbox page. Local sysops cannot change the content model or delete it.

We can change the model, but I don't know if the existing content would be kept.